The present invention relates generally to the field of HID lamps, and more particularly to HID lamps with short-term color stability.
Recently a new generation of ceramic high intensity discharge lamps that are long and thin have been disclosed. In this regard, see WO 00/45419. These lamps have higher efficacy compared to the original ceramic high intensity discharge lamps (aspect ratio close to 1) and better maintenance. One issue with these long and thin lamps is that the color temperature can vary by hundreds of degrees Kelvin over a period of hours. This variation can be quite noticeable to the end user, especially when multiple lamps are utilized.
The design of ceramic high intensity discharge lamps includes the main cylindrical lamp discharge vessel with smaller diameter ceramic feedthoughs at each end. The electrodes pass through the feedthroughs into the main cylindrical lamp discharge vessel. The metal halide chemistry (also called condensate) that is added to the lamp mostly remains in the main cylinder, but some of it collects in the annular space between the electrodes and the smaller diameter inner wall of the feedthroughs. The metal halide condensate that vaporizes in the lamp and enters the discharge largely determines the spectrum of the discharge and hence its color temperature. The composition of the vapor that enters the discharge is determined both by the temperature of the condensate and its chemical composition. Two typical components of the metal halide chemistry are sodium iodide and cerium iodide. The ratio of these two metals in the discharge will strongly influence the spectrum and the color temperature. A discharge richer in cerium will have a high color temperature, while one richer in sodium will have a lower color temperature. The lamp tends to cycle between these two extremes. When the discharge is rich in sodium the current is high and the voltage is low. The discharge is more diffuse radially. When the discharge is rich in cerium the current is lower and the voltage is higher. The discharge is more constricted radially. Through a process that is not well understood the lamp cycles between these extremes. The presence of condensate in the smaller diameter feedthrough seems to be involved in this color temperature instability. The observed time constants of many hours suggest a thermal process involving the condensate.
There are many variables in designing ceramic high intensity discharge lamps. In addition to the overall dimensions of the lamp, the size and insertion length of the electrode (tip to bottom distance) can be controlled. The length of the feedthrough can also be adjusted. This feedthrough length adjustment is the approach taken to control color temperature by Matsushita Electronics and described in European Patent Application EP 1058288.
Briefly, the present invention comprises, in a first embodiment, an HID lamp with short-term color temperature stability, comprising: a high intensity discharge lamp with a discharge vessel and electrodes; and a filling within the discharge vessel containing a metal halide dose of at least 20 mg/cc, where volume is defined as the volume of the main cylindrical section of the discharge vessel.
In a further aspect of the present invention, the filling within the discharge vessel contains Hg of at least 20 mg/cc, where volume is defined as the volume of the main cylindrical section of the discharge vessel.
In a further aspect of the present invention, the color temperature is stabilized independent of the shape and frequency of the current and voltage waveforms that drive the HID lamp.
In a further aspect of the present invention, the color temperature is stabilized independent of the shape and frequency of the current and voltage waveforms that drive the HID lamp.
In a further aspect of the present invention, the HID lamp discharge vessel has an inner length/inner diameter ratio equal to or greater than 3.
In a further aspect of the present invention, the HID lamp discharge vessel has an inner length/inner diameter ratio equal to or greater than 4.75.
In a further aspect of the present invention, the filling includes cerium.
In a further aspect of the present invention, the filling within the discharge vessel contains a metal halide dose of at least 24 mg/cc, where the volume is defined as the volume of the main cylindrical section of the discharge vessel.
In a further aspect of the present invention, the filling within the discharge vessel contains Hg of at least 24 mg/cc, where the volume is defined as the volume of the main cylindrical section of the discharge vessel.
In a further embodiment of the present invention, a method is provided for obtaining short-term color stability in an HID lamp, comprising the steps of: providing a high intensity discharge lamp with a discharge vessel and electrodes; and filling the discharge vessel with a metal halide dose to at least 20 mg/cc, where the volume is defined as the volume of the main cylindrical section of the discharge vessel.